But structure is also musical material! The fact that there is a tight mirroring structure for me is already a lot of material shared.
I disagree about the first sentence, but even if I didn't, the structure isn't really as "tight(ly) mirroring" as you claim. The idea with the two halves is interesting, but to get to that point you're basically flattening a lot of it down to "here's a vocal section, then comes a wanky instrumental section". But the two main vocal sections are quite different in structure themselves - the first is verse, verse, chorus, unrelated new verse, chorus, new vocal section that isn't repeated. The second is three verses into a new vocal section that isn't repeated. So they're not a mirror of each other structurally. The instrumental bits that follow the vocal parts also quite different in structure. Yes, lyrically the song has a cool "questions - development - answers" structure in the middle and these bits are bookended by the same riff, but despite your claim, the ending doesn't actually connect to the opening musically. Honestly, I don't even really see how it connects to the rest of the song lyrically.
Rather than anything "coming full circle" it all just seems tagged on to have a big ending section. Sure, they *named* the instrumental opening theme "paradoxe de la lumière noire" which you can connect to the lyrics of the ending, but that's just a sleight of hand.
But if someone can point out shared musical material that I've missed, I'm all ears! I enjoy these discussions.